1. Field of the Invention
This invention relates to the hydrodesulfurization of petroleum oils, preferably those containing residua hydrocarbon components, and having a significant metals and sulfur content. More particularly the invention relates to an improved method for desulfurization which allows for significantly longer operating cycles and/or reduced operating severity; a reduced operating severity produces a correspondingly reduced investment and operating cost.
2. Description of the Prior Art
The benefits of purifying petroleum fractions through hydrogen processing are well known. Due primarily to a lack of inexpensive hydrogen and the relatively high pressures required, the process did not develop commercially to a substantial extent until the advent of catalytic reforming, which produced hydrogen rich off-gas as a byproduct. The functions of hydrogen treating are primarily removal of sulfur compounds, nitrogen compounds and other impurities; hydrogen saturation of olefins and/or aromatics; and mild hydrocracking. Certainly one of the most commercially important functions is that of sulfur removal.
It has been proposed to improve the salability of high sulfur content, residual-containing petroleum oils by a variety of hydrodesulfurization processes. However, difficulty has been experienced in achieving an economically feasible catalytic hydrodesulfurization process, because notwithstanding the fact that the desulfurized products may have a wider marketability, the manufacturer may be able to charge little or no additional premium for the low sulfur desulfurized products, and since hydrodesulfurization operating costs have tended to be relatively high in view of the previously experienced, relatively short life for catalysts used in hydrodesulfurization of residual-containing stocks. Short catalyst life is manifested by inability of a catalyst to maintain a relatively high capability for desulfurizing chargestock with increasing quantities of coke and/or metallic contaminants which act as catalyst poisons. Satisfactory catalyst life can be obtained relatively easily with distillate oils but is especially difficult to obtain in desulfurizing petroleum oils containing residual components, since the asphaltene or asphaltic components of an oil, which tend to form disproportionate amounts of coke, are concentrated in the residual fractions of a petroleum oil, and since a relatively high proportion of the metallic contaminants that normally tend to poison catalysts are commonly found in the asphaltene components of the oil.
The most common desulfurization catalyst is cobalt molybate on an alumina base, however any of the Group VIB and Group VIII metals may be employed as a hydrogenation component on a suitable refractory base material. Typical operating condition ranges for resid and/or crude desulfurization are a temperature of about 650.degree. to 850.degree.F., a space velocity of about 0.1 to 5.0 L.H.S.V., a pressure of about 500 to 3,000 p.s.i.g. and a hydrogen circulation of about 1,000 to 15,000 s.c.f./bbl of feed.
The removal of metals in hydrodesulfurization operations is generally undesirable since the typical desulfurization catalyst such as cobalt molybate on an alumina base is poisoned by metals deposition. In the past, this type of process has been operated in such a manner as to maintain a substantially constant conversion or severity, that is level of sulfur removal. In order to achieve this desired level of sulfur removal, the operating conditions were steadily increased in severity to compensate for the gradual catalyst deactivation primarily due to metals poisoning and coking.
Process severity may be described as being directly related to temperature and pressure, and inversely proportionated to the space velocity of the process. Thus in order to increase severity, one might increase pressure and/or temperature or decrease the space velocity. As most process units are sized based on throughput and pressure, neither the contact time nor the pressure can be significantly increased, therefore severity is typically increased through a temperature increase. Thus most residua desulfurization reactors are initially operated at a "start of run" temperature of about 650.degree.F to 750.degree.F. As the desulfurization catalyst activity decreases due e.g., to metals deposition and coke formation, the reaction severity is increased by increasing the temperature, so as to maintain a desired substantially constant sulfur removal level. "End of run temperature" is typically about 800.degree.F and is reached when the catalyst activity has been significantly decreased, e.g., due to metals poisoning and coking. Were it not for such metals poisoning of the desulfurization catalyst, the operating cycles could be lengthened, or the severity could be reduced (lower temperatures and/or pressures and/or increased space velocities).
At the present time, and certainly for several years into the foreseeable future, low sulfur fuel oils are and will be in critical demand. At the same time that recent legislation has reduced the allowable sulfur levels in fuel oils, the overall demand for fuel oils has increased markedly. As a consequence, the need for desulfurized petroleum products such as fuel oils has been doubly increased.
An object of this invention is to provide a method of hydrodesulfurization of metals and sulfur containing petroleum oils, preferably those containing residua hydrocarbon fractions, whereby the operating cycle, that is, number of days on stream, for such a process may be significantly increased, without any significant decrease in sulfur removal. An additional object of this invention is to provide a method of hydrodesulfurization of petroleum oils, preferably those containing residua hydrocarbon fractions, whereby the severity of the operation and its attendant investment and operating cost are decreased. That is, pressure and/or temperature might be reduced and/or the space velocity increased, without any significant decrease in sulfur removal. Another object of this invention is to provide a method of hydrodesulfurization whereby the metals poisoning of the desulfurization catalyst is significantly reduced. Other and additional objectives of this invention will become obvious to those skilled in the art following a consideration of the entire specification including the drawing and claims.